Volume 44, Issue 1, Pages 9-16 (October 2011) RIPK-Dependent Necrosis and Its Regulation by Caspases: A Mystery in Five Acts  Douglas R. Green, Andrew Oberst, Christopher P. Dillon, Ricardo Weinlich, Guy S. Salvesen  Molecular Cell  Volume 44, Issue 1, Pages 9-16 (October 2011) DOI: 10.1016/j.molcel.2011.09.003 Copyright © 2011 Elsevier Inc. Terms and Conditions

Figure 1 A Mysterious Death (A) Schematic of apoptotic death pathways. Extrinsic signals through death receptors like CD95 or intrinsic signals that activate proapoptotic Bcl-2 members like Bid and Bim lead to activation of downstream effector caspases (caspase-3). Loss of proapoptotic molecules lead to a range of phenotypes, including embryonic lethality associated with exencephaly (caspase-9, APAF-1, or caspase-3 knockouts) and lymphoaccumulative and autoimmune disorders (CD95/CD95L or Bim knockout mice). (It should be noted, however, that while cells lacking downstream elements of the mitochondrial pathway do not undergo apoptosis via this route, cells may nevertheless die by a caspase-independent mechanism). Additionally, animals deficient in certain proapoptotic molecules (i.e., Bim or Bax knockouts) display enhanced oncogenesis. Cells from animals where proapoptotic proteins are deleted are generally resistant to apoptosis, while cells from animals missing antiapoptotic proteins (such as Bcl-2 and Bcl-xL) have increased sensitivity to apoptotic stimuli. (B) Ablation of caspase-8 leads to embryonic lethality circa e10.5. The simultaneous deletion of caspase-8 and RIPK3 or FADD together with RIPK1 rescue embryonic development, suggesting that FADD and caspase-8 act in concert to provide survival signals which may inhibit RIPK function during development. Molecular Cell 2011 44, 9-16DOI: (10.1016/j.molcel.2011.09.003) Copyright © 2011 Elsevier Inc. Terms and Conditions

Figure 2 Caspase 8 and FLIP—the Twins of Death and Protection (A) Caspase-8 consists of a prodomain, followed by the large and the small protease subunits, separated by linker regions. The prodomain has two death “folds” named death effector domains (DEDs, light blue), through which caspase-8 is recruited and dimerized. Upon dimerization, caspase-8 becomes proteolytically active and processes itself in specific cleavage sites present in the linker regions. The cleavage between the large and small subunit contributes to the stabilization of the molecule and is essential to induction of cell death but is dispensable for RIPK-necrosis inhibition. (B) FLIP has a similar structure to that of caspase-8, but it lacks the catalytic cysteine. Although it can act as a dominant negative for caspase-8-induced death when overexpressed, FLIP acts in concert with caspase-8 to inhibit RIPK-induced necrosis. Molecular Cell 2011 44, 9-16DOI: (10.1016/j.molcel.2011.09.003) Copyright © 2011 Elsevier Inc. Terms and Conditions

Figure 3 Our Protector, but How? Caspase-8-FLIP complex blocks RIPK-dependent necrosis, although the precise mechanism behind this inhibition is still unknown. RIPK1 and RIPK3, as well as upstream deubiquitinase CYLD, are caspase-8 cleavage targets, suggesting that cleavage of any of these proteins might be critical for necrosis inhibition. However, as little is known about what is downstream RIPK1/RIPK3 activation, other targets for caspase-8 may yet be implicated. Molecular Cell 2011 44, 9-16DOI: (10.1016/j.molcel.2011.09.003) Copyright © 2011 Elsevier Inc. Terms and Conditions